An enzyme could make a dream come true for the energy industry: it can efficiently produce hydrogen using electricity, and it can also generate electricity from hydrogen. The enzyme is protected by incorporating it into a polymer. An international research team with significant participation of scientists from the Technical University of Munich (TUM) presented the system in the scientific journal Catalysis of nature.
Fuel cells turn hydrogen into electricity, while electrolysers use electricity to split water to produce hydrogen. Both need platinum, a rare and therefore expensive precious metal, as a catalyst. Nature created a different solution: enzymes, called hydrogenases. They catalyze the conversion of hydrogen very quickly and with almost no loss of energy.
However, in the past, these biocatalysts were not considered suitable for industrial use due to their high sensitivity to oxygen. Now, a research team from the Technical University of Munich (TUM), the Ruhr-Universität Bochum (RUB), the National Center for Scientific Research (CNRS) in Marseille and the Max-Planck Institute for conversion chemical energy has succeeded in integrating the sensitive enzymes into a protective polymer in a way that makes them viable for use in the technical conversion of hydrogen.
Sustainability vs activity
“When sensitive hydrogenases are integrated into appropriate polymers, they continue to work for several weeks, even in the presence of oxygen,” explains Nicolas Plumeré, professor of electrobiotechnology at TUM Campus Straubing for biotechnology and sustainable development. “Without this protection, they lose their activity in a matter of minutes.”
The incorporation of hydrogenases into polymers whose side chains can transfer electrons, called redox polymers, nevertheless had two decisive drawbacks: a high level of resistance counterbalanced the flow of electrons through the redox polymer. This required the investment of energy which was then lost in the form of heat. And the integrated hydrogenases have completely lost their ability to generate hydrogen.
Fine tuning potential
With intelligent selection of the right polymer side chains, the research team has now succeeded in tuning the redox potential of the polymer such that only a small surge is needed to overcome the resistance.
More detailed research then revealed that the side chain potential had shifted slightly to positive values due to the embedding in the polymer matrix. In another attempt, they used a side chain with a corresponding negative potential. This trick was the breakthrough: the hydrogenase was now able to catalyze the reaction in both directions without wasting energy.
Biocatalyst for hydrogen conversion
Using this system, the research team then built a fuel cell, in which oxygen is reduced by the bilirubin oxidase enzyme from the bacterium Myrothecium verrucaria, while the hydrogenase embedded in the polymer film oxidizes it. hydrogen from the bacterium desulfovibrio desulfuricans, generating electricity in the process. .
The cell reaches a value, with an open circuit voltage of 1.16 V, the highest ever measured for a system of this type and close to the thermodynamic maximum. With three milliamps per square centimeter, the cell achieves a very high power density for biological cells at the same time.
The system can also be used for the reverse reaction, producing hydrogen by consuming electrons: the energy conversion efficiency is close to 100%, even with power densities above four milliamps per square centimeter.
Master plan for new biocatalysts
“Reducing energy loss has two decisive advantages,” explains Nicolas Plumeré. “First, it makes the system much more efficient; second, the heat generated in a fuel cell at high performance levels would be a problem for biological systems.”
In order to make their system competitive with systems using platinum-based catalysts, the team’s ongoing research is now focused on improving the stability of hydrogenases at higher power densities.
In addition, the results can also be transferred to other highly active but sensitive catalysts for energy conversion and electrosynthesis. The direct targets here are primarily the carbon dioxide reducing enzymes that can use electricity to produce liquid fuels or intermediates from carbon dioxide.
The research was funded by a start-up grant from the European Research Council (ERC), by the National Center for Scientific Research (CNRS) and Aix Marseille University, the German Research Foundation (DFG) co-funded with the Agency National Research, the DFG priority program “Iron – Sulfur for life” (SPP 1927), the company Max Planck and in the case of the RESOLV center of excellence of the Federal Ministry of Education and Research (BMBF) within the framework of the excellence strategy of the Federal Government and German Federal States.